1. Field of the Invention
The present invention relates to novel chiral dopants for liquid crystals, and to liquid-crystalline mixtures which contain such dopants, and to their use for optical and electrooptical purposes.
2. Description of the Invention
Liquid-crystal materials for electrooptical display devices frequently contain one or more optically active additives for inducing a chiral structure. For example, a nematic liquid crystal is preferably doped with an optically active additive for use in display devices having a twisted nematic structure, for example in order to avoid reverse twist in TN (twisted nematic) cells or in order to achieve adequate twisting in cells having a highly twisted nematic structure, such as STN (supertwisted nematic) cells, SBE (super birefringence effect) cells or OMI (optical mode interference) cells. Furthermore, cholesteric liquid crystals for phase-change cells can preferably comprise a nematic base material and one or more optically active dopants, and ferroelectric liquid crystals for display devices based on chiral, tilted, smectic phases can preferably comprise a material having a tilted smectic phase and one or more optically active dopants.
The electrooptical response curves of liquid-crystal display devices are temperature dependent, which is particularly disadvantageous when operated in multiplex mode. However, it is known that this temperature dependence can be compensated, at least partially, by adding a chiral dopant which induces a pitch which decreases with increasing temperature. An inverse temperature dependence of this type has only been found for a few compounds. However, it can also be achieved by using at least two chiral dopants which have different relative temperature dependences and induce different twist directions (DE-A-2827471 corresponding to U.S. Pat. No. 4 264 148). However, this usually requires a relatively high proportion of chiral dopants.
Cholesteric liquid crystals reflect light in a wavelength range for which the wavelength is approximately equal to the helix pitch. The spectral width of the reflected light can be varied by a suitable choice of the liquid crystal. The reflected light is fully circular-polarised. The direction of rotation of the reflected light depends on the direction of rotation of the cholesteric helical structure. The light circular-polarised in the opposite direction is transmitted without absorption. These properties can be utilized to produce optical filters, polarizers, analyzers etc. Furthermore, cholesteric liquid crystals have also occasionally be used for thermochromic applications and in cosmetic preparations.
Cholesteric liquid crystals for the above applications can preferably comprise a nematic or cholesteric base material and one or more chiral dopants, which allows the desired helix pitch to be established.
In order to achieve cholesteric mixtures having a pitch in the range of the wavelength of visible light, the chiral dopants should have high a helical twisting power and be readily soluble in customary liquid-crystal materials. In addition, the chiral dopants should have adequate stability, be readily compatible with the mesophase type of liquid-crystal material and should not restrict the mesophase range excessively. Such properties are also desirable for chiral dopants for achieving the twisted nematic structures mentioned at the outset, since their proportion could be kept so low that the properties of the liquid-crystal material are affected only insignificantly.